This invention relates to structural, load carrying members and a process for constructing such members. More particularly, it is directed to a structural member formed of bonded, elongated wood strands and a process for constructing same.
In the design of load carrying members it is well understood among engineers that when such members are subjected to bending, for example a simply supported beam carrying a load at its center, a number of stresses are introduced in the beam and the intensity of these stresses varies with the position in the beam. When a simply supported beam is supporting a load the intensity and the direction of stresses within the beam vary from the top to the bottom of the beam. The top of the beam or the concave surface is subjected to compressive stresses which are maximum at the surface of the beam and gradually decrease to zero at the neutral axis of the beam. Tensile stresses develop on the convex side of the beam. As in the case of the compressive stresses the tensile stresses are maximum at the surface of the beam, gradually decreasing to zero at the neutral axis. The intensity of the tensile and compressive stresses will also vary from one end of the beam to the other. A simply supported beam with a load at the center will have maximum tensile and compressive stresses at the center of the length, gradually decreasing to zero at the supports. Other loading conditions and different support situations will influence the distribution of stresses throughout the beam.
Shear stresses also develop in structural members subjected to flexure. In the case of simply supported beams of uniform cross section, the maximum shear stress is reached at the neutral axis over the supports.
Loaded cantilever beams develop stresses as found in simply supported beams with tensile stresses developing on the convex side of the flexed beam and compressive stresses developing on the concave side of the beam. Again shearing stress is created at the neutral axis.
In the case of cantilevered beam maximum compressive and tensile stresses are found at the support with maximum shear at the free end of the member.
Structural members containing apertures will also develop other stresses around or near the openings.
In constructing structural members it is logical to align and position the strongest elements of the member at the point of highest stress. In solid wood beams this is accomplished by aligning the grain structure parallel to the length of the beam thereby providing maximum resistance to the tensile and compressive forces. Natural wooden beams or conventionally laminated wooden beams, however, do provide lowest shear resistance at the neutral axis because of the grain direction in the wood. Under conventional design, on the other hand, wood beams are thick enough and dense enough to provide the needed shear strength. In the case of the oriented wood strand beam, the subject of this invention, the wood strands are oriented parallel to the length of the beams in the flanges where the compressive and tensile stresses will be at their greatest. At the neutral axis which is located in a line along the web, the strands can be oriented in a direction that will give the greatest resistance to shear stresses that might develop in the structural member. Similarly strands can be suitably oriented to withstand stresses that are created around appropriately designed apertures.
Recent developments in the field of wood strand technology have led to the development of reconstituted wood products that have the characteristic of strands therein aligned substantially parallel to one another and to one side of the panel. It has been found possible, with oriented wood strands compressed together and bonded with an appropriate resin, to form a beam or load carrying member that provides bearing characteristics somewhat similar to that of presently known wood load carrying members.
U.S. Pat. No. 3,115,431 to Stokes shows oriented wood particleboard composed of layers, with outer layers running parallel to the length of the panel, and inner layers perpendicular or random in orientation. This disclosure is concerned primarily with the construction of plywood-like panels rather than beams or other specially designed load carrying members. U.S. Pat. No. 2,960,423 to Kreibaum discloses a panel made from randomly oriented particles with veneer surface having oriented grain. The oriented surfaces however are wider than the board thickness.
The product of the present invention is constructed completely of bonded elongated wood strands with integral flanges along the longitudinal side edges thereof. Strands within the flanges are oriented longitudinally along the edges, the strands located within the web being randomly or perpendicularly oriented relative to the flange strands. The density of the mass in the center web may also be less than the density of the material in the flanges. It is intended that the present product be manufactured preferably as an elongated beam of rectangular cross section. The rectangular shape enables formation of a plurality of such beams or members from a single panel formed by conventional particleboard forming machinery. The present process therefore is comprised of steps including placing parallel strips of oriented strands on a supporting surface or "caul" and depositing webs of strands between the parallel strips to form interfaces at junctures of the webs and strips. The webs and strips are deposited within a rectangular area similar in shape to a conventional panel of particleboard. A conventional press may be utilized to form or compress the strips bonding them together to form a conventional sized panel. Once formed, the panel may be cut longitudinally, dividing the strips of longitudinally oriented strands to form individual support members, each comprising a central web section bounded by flanges of longitudinally-oriented structural strands.